This invention relates to ceramic or ceramic-like coatings and methods of coating substrates, and more particularly to the low temperature formation of monolayer and multilayer ceramic or ceramic-like coatings.
It is desirable for electronic circuits and devices and other nonmetallic substrate materials to be serviceable under a range of environmental conditions. Further, many of the uses for electronic devices today places a premium on size and weight. For example, electronic circuits used in spacecraft, satellites, and aircraft need not only to be able to withstand a wide variety of environmental conditions, but also must be compact and lightweight in use. In order to protect such devices and substrates from heat, moisture, and abrasive forces, the art has resorted to a number of methods to coat the devices and substrates to prevent, or at least minimize, the exposure of the devices or substrates to these environmental conditions.
Early attempts at protecting electronic circuitry included potting the circuits in polymeric resins. However, these techniques added considerable thickness and weight to the circuits. Also, the polymeric coatings tended to absorb moisture from the environment which could eventually lead to damage or failure of the circuits. Presently, some circuits are contained in ceramic packages to protect them from environmental exposure. While the ceramic packages are relatively secure, they add a substantial amount of thickness and weight to the circuit. Further, they are relatively expensive to fabricate as the package typically must be evacuated of air, the device inserted, and then the package sealed.
Common causes for the failure of electronic devices include the formation of microcracks or voids in the surface passivation of the device, such as a semiconductor chip, permitting the introduction of impurities from the environment. For example, sodium (Na+) and chloride (Cl-) ions may enter electronic devices and disrupt the transmission of electrical signals. Additionally, the presence of moisture and/or volatile organic chemicals may also adversely affect the performance of electronic devices. A single coating material or layer may be insufficient to meet the ever increasing demands placed on the material by the electronics industry. Several coating properties such as microhardness, moisture resistance, ion barrier, adhesion, ductility, tensile strength, and thermal expansion coefficient matching must be achieved through the use of a number of thin protective layers on the electronic device.
More recently, lightweight monolayer and multilayer ceramic coatings have been developed for coating electronic devices. For example, Haluska et al, in U.S. Pat. Nos. 4,753,855 and 4,756,977, teach the formation of ceramic coatings by producing a solvent mixture of a hydrogen silsesquioxane resin alone or in combination with a metal oxide precursor which is then coated onto the surface of an electronic device. The coating is ceramified at temperatures between about 200.degree. to 1000.degree. C. to form a silicon dioxide-containing ceramic coating. Additional coating layers of ceramic materials are also taught to provide additional protection and coating properties. These additional layers may comprise additional ceramic or ceramic-like coatings containing silicon, silicon and carbon, or silicon, carbon, and nitrogen.
While the techniques taught in the above two Haluska et al patents are effective to provide silicon dioxide-containing ceramic coatings at from about 200.degree. to 1000.degree. C. the ceramification of the hydrogen silsesquioxane resin is quite slow at the lower end of the temperature range so that, for practical production purposes, temperatures of at least 400.degree. C. need to be utilized. Although some electronic devices can withstand these higher temperatures, other devices, such as those containing gallium arsenide components, may not be able to withstand temperatures of 400.degree. C. and above. Additionally, the time period for which the device is exposed to a given temperature may be critical in that short exposure times at a given temperature may be acceptable, while longer exposure times at the same temperature may cause degradation and failure of the electronic device.
Accordingly, there still exists a need in this art for a thin, lightweight ceramic coating and method of application which can be utilized on heat sensitive electronic devices and other heat sensitive substrates. Further, the need still exists for a ceramification procedure which proceeds rapidly at temperatures below about 400.degree. C.